Thread exception when calling close on client after watcher is started

**Description**:
The callback thread throws an exception when calling close on client if a watch has been started. Even if the watch is canceled the thread remains and throws a ValueError at etcd3/watch.py line 126.

**Code to reproduce**:
```
import etcd3

client = etcd3.client()
watch_id = client.watch_prefix('/')
client.cancel_watch(watch_id)

client.close()
```

**Error message**:
```
Exception in thread etcd3_watch_7f99646ef9b0:
Traceback (most recent call last):
File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
self.run()
File "/usr/lib/python3.6/threading.py", line 864, in run
self._target(*self._args, **self._kwargs)
File "/usr/local/lib/python3.6/dist-packages/etcd3/watch.py", line 126, in _run
metadata=self._metadata)
File "/usr/local/lib/python3.6/dist-packages/grpc/_channel.py", line 1084, in __call__
event_handler, self._context)
File "/usr/local/lib/python3.6/dist-packages/grpc/_channel.py", line 1171, in create
operationses_and_tags, context)
File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 478, in grpc._cython.cygrpc.Channel.integrated_call
File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 296, in grpc._cython.cygrpc._integrated_call
File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 222, in grpc._cython.cygrpc._call
File "src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx.pxi", line 262, in grpc._cython.cygrpc._call
ValueError: Cannot invoke RPC: Channel closed!
```

**python version**:
Python 3.6.9

**pip freeze**:
asn1crypto==0.24.0
coverage==5.0.3
crc32c==2.0
cryptography==2.1.4
etcd3==0.11.1
grpcio==1.26.0
idna==2.6
keyring==10.6.0
keyrings.alt==3.0
numpy==1.18.1
protobuf==3.11.2
pycrypto==2.6.1
pygobject==3.26.1
pyxdg==0.25
PyYAML==5.3
rdbd==0.0.1
SecretStorage==2.3.1
six==1.13.0
tenacity==6.0.0

**etcd --version**:
etcd Version: 3.2.17
Git SHA: Not provided (use ./build instead of go build)
Go Version: go1.10
Go OS/Arch: linux/amd64

**operating system**:
Docker version 19.03.5, build 633a0ea

**Dockerfile**
FROM ubuntu:18.04

RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install --assume-yes \
etcd \
python3 \
python3-pip \
supervisor && \
mkdir /python

ENV DEBUG true
ENV ETCD_NAME provider
ENV ETCD_DATA_DIR /var/lib/etcd
ENV ETCD_INITIAL_CLUSTER_STATE new
ENV ETCD_INITIAL_CLUSTER_TOKEN etcd-cluster-01
ENV ETCDCTL_API=3
ENV CLUSTER_SEED_NODES node1,node2,node3

**etcd_start.sh**
```
#!/bin/bash

set -x
CURRENT_IP=$(hostname -i)
CLUSTER_SEED_NODES=${CLUSTER_SEED_NODES:-localhost}

INITIAL_CLUSTER=""
for NODE in $(echo ${CLUSTER_SEED_NODES} | sed 's/,/ /g'); do
NODE_ENTRY="${NODE}=http://${NODE}:2380"
if [[ -z "${INITIAL_CLUSTER}" ]]; then
INITIAL_CLUSTER="${NODE_ENTRY}"
else
INITIAL_CLUSTER="${INITIAL_CLUSTER},${NODE_ENTRY}"
fi
done

/usr/bin/etcd \
--name $(hostname) \
--initial-advertise-peer-urls "http://${CURRENT_IP}:2380" \
--listen-peer-urls "http://${CURRENT_IP}:2380" \
--listen-client-urls "http://${CURRENT_IP}:2379,http://127.0.0.1:2379" \
--advertise-client-urls "http://${CURRENT_IP}:2379" \
--initial-cluster-token etcd-cluster \
--initial-cluster ${INITIAL_CLUSTER}
```

Inizia da etcd3/watch.py in _run, intorno alla riga 126, poi segui i percorsi di watch_prefix(), cancel_watch() e client.close() usando la riproduzione fornita. Verifica come viene arrestato il thread del watcher prima della chiusura del canale gRPC. Il lavoro è completato quando l’esecuzione dell’esempio dopo l’annullamento del watch e la chiusura del client non produce più l’eccezione Channel closed.

Scritto dal modello di indicizzazione a partire dal testo della issue.